The fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a process chamber. Various recipes are used in the fabrication process, and many discrete processing steps, where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., can be required. The steps used, their particular sequence, and the materials involved all contribute to the making of particular devices.
Accordingly, wafer fabrication facilities are commonly organized to include areas in which chemical vapor deposition, plasma deposition, plasma etching, sputtering and other similar gas manufacturing processes are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition, must be supplied with various process gases. Pure gases must be supplied to the tools in contaminant-free, precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas box. The gas box delivers contaminant-free, precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool. The gas box, or gas metering system includes a plurality of gas paths having gas metering units, such as valves, pressure regulators and transducers, mass flow controllers and filters/purifiers. Each gas path has its own inlet for connection to separate sources of gas, but all of the gas paths converge into a single outlet for connection to the process tool.
Sometimes dividing the combined process gases equally among multiple process chambers, or among separate portions of a single process chamber, is desired. In such cases, the single outlet of the gas box is connected to secondary flow paths. To insure that the primary flow of the outlet of the gas box is divided equally among the secondary flow paths, flow restrictors are placed in each secondary flow path.
What is still desired, however, is a mass flow ratio system and method for dividing a single flow into a desired ratio of two or more flows. Preferably, the system and method will operate independently of the gas or gases controlled. In addition, the system and method preferably will not disturb the performance of any upstream mass flow controllers.